International consensus on the standardization of sodium and potassium measurements by ion-selective electrodes in...

The International Federation of Clinical Chemistry (IFCC) and the National Committee for Clinical Laboratory Standards (NCCLS) are about to recommend to adjust sodium and potassium measurements by ion-selective electrodes in undiluted samples to the amount of substance concentration in the sample as determined, e.g., by flame atomic emission spectrometry. The adjustment is only valid in case of normal standardized sera (or plasma), implying "normal" water concentration (normal concentration of proteins, lipids or other macromolecules), "normal" binding of the pertinent electrolytes and "normal" coefficient of activity. If these criteria are not met, results obtained by "adjusted" ISE's will differ from total molar concentration. That is: in individual samples of patients results from ISE's and total molar concentration will differ unpredictably. It forced IFCC to propose new quantities for the measurements by adjusted ISE's: ionized sodium and ionized potassium. The reference interval for ionized sodium and ionized potassium is identical to the pertinent reference interval for molar concentration of total sodium and total potassium, but it is in contrast independent from water concentration and valid, e.g., in hypoproteinaemia as well as in hyperlipaemia or hyperproteinaemia. Accuracy control of ionized sodium and ionized potassium based on reference method values is hampered by abnormal water concentration and inadequate properties of the matrix of many control sera. Alternative approaches how to report measurements by ISE's in undiluted samples, such as activity or free molal concentration are discussed with their pros and cons regarding accuracy control by reference method values. The need for appropriate control materials with a matrix similar to native human sera is stressed.